Uranium etchant and method

ABSTRACT

METALLIC URANIUM AND ALLOYS THEREOF ARE PROVIDED WITH UNIFORMLY ETCHED SURFACES FOR FACILITATING THE RECEPTION OF ADHERENT PROTECTIVE COATINGS. THE ETCHANT CONSISTS OF AN AQUEOUS SOLUTION OF EITHER FERRIC CHLORIDE OR FERRIC CHLORIDE AND NICKEL CHLORIDE.

March 30,",1971 WALDRQP ET AL 3,573,120

URANIUM ETCHANT AND METHOD Filed Sept. 6. 1968 Fe C| '6H conc.(moles/liter) URANIUM REMOVAL IN ONE FI'NUTE (mils) Fig.1

2R 4 ,2 i 3 5w m; 2 2 3% 1 2; 4/ :0- 0

TEMPERATURE (*c) Fig.

1N VENTORS.

Forrest B. Wq/drop BY Max J. Bez/k United States Patent 01' lice U.'S.Cl. 15618 2 Claims ABSTRACT OF THE DISCLOSURE Metallic uranium andalloys thereof are provided with uniformly etched surfaces forfacilitating the reception of adherent protective coatings. The etchantconsists of an aqueous solution of either ferric chloride or ferricchloride and nickel chloride.

Structures or articles of metallic uranium and uranium alloys arecustomarily provided with a protective sheath or coating, usually anelectro-deposit of nickel, for minimizing the health hazards associatedwith the handling of uranium and for protecting the structures fromdeleterious corrosion. The preparation of the uranium surfaces forreceiving a firmly adherent coating necessarily includes the importantstep of contacting the surfaces with an etchant for roughening orotherwise preparing these surfaces to facilitate the formation of atenacious mechanical bond between the coating and the uranium. Theetchant should be capable of uniformly etching the entire contactedsurfaces while effecting minimal material removal.

The prior art is replete with uranium etching solutions, such as, forexample, an aqueous solution containing nickel chloride and nitric acid,an ethyl alcohol-strannous; chloride solution and an anodic pickle in aphosphoric acid-hydrochloric acid solution, a nickel chloride-nitricacid-hydrochloric acid solution, and a stannous chloridenitric acidsolution. While these previously known etchants have provided uraniumwith surfaces receptive to the protective coating, they each, for onereason or another, exhibit shortcomings or drawbacks which detract fromtheir usefulness. For example, the nickel chloride-nitric acidwateretchant suffers a changing etching rate due to the variance in thechloride and nitrate contents which is caused by the interaction of thetwo ions. This interaction occurs even when the etchant is not beingused. Relatively rapid etching rates encountered by using some of theprevious etchants create an undesirable condition since very precisetiming schedules are required to prevent excessive etching which resultsnot only in excessive surface roughness and material removal, but alsoin non-uniformly etched surfaces. Another disadvantage of rapid etchingrates is due to significant temperature increases during the etchingoperation that cause the uranium to heat up and continue to etch withadhering etchant when removed from the etchant bath and before the waterrinsing step. The useable life of previous etchants as Well as theetching rates of these etchants is significantly influenced by theuranium concentrations in the etchant since previous etchants withuranium concentrations of over about 6 ounces per gallon of the etchantwere rendered virtually useless.

It is the aim of the present invention to provide an etching solutionfor uranium and uranium alloys which substantially minimizes or obviatesthe above and other shortcomings or drawbacks suffered by previouslyknown etchants. In accordance with this invention structures of metallicuranium or uranium alloys are uniformly etched with minimal materialremoval in a novel etching solution 3,573,129 Patented Mar. 30, 1971consisting of an aqueous solution of ferric chloride or ferric chlorideand nickel chloride.

An object of the present invention is to provide a new and improvedetching solution for etching uranium or uranium alloys in preparationfor the reception of protective coatings particularly electro-depositednickel plate.

Another object of the present invention is to provide for the etching ofuranium and uranium alloy surfaces in a uniform manner and with minimalmaterial removal.

A further object of the present invention is to provide a uranium anduranium alloy etchant consisting of an aqueous solution of ferricchloride or ferric chloride and nickel chloride and capable of providinga relatively slow etching rate for permitting better control over theetching operation.

Other and further objects of the invention will be obvious upon anunderstanding of the following description or will be indicated in theappended claims, and various advantages not referred to herein willoccur to one skilled in the art upon employment of the invention inpractice.

In the accompanying drawing:

FIG. 1 is a graph showing the etching rates on metallic uranium whenaqueous ferric chloride solutions of various concentrations areemployed; and

FIG. 2 is a graph showing the effect of temperature on the rate ofetching metallic uranium in a 5-molar ferric chloride solution.

Described generally, the present invention relates to a new and improvedchemical solution for etching uranium or uranium alloys for the purposeof preparing such alloys for the reception of a protective coating,particularly electrolytically or electrolessly deposited nickel. Theetching solution envisioned herein consists of a ferric chloridewatersolution which has been found to effect uniform etching of exposeduranium or uranium alloy surfaces at a moderate or relatively slow ratefor providing a desirable surface to which the nickel coating willadhere tenaciously. The etching solution is prepared by admixing ferricchloride hexahydrate (FeCl -6H O) in water. An alternative aqueoussolution can be prepared by utilizing a mixed chloride solutionconsisting of nickel chloride (NiCl and the ferric chloride hexahydrate.

As shown in FIG. 1, the concentration of ferric chloride hexahydrate inmoles per liter of water has a considerable influence upon the etchingrate of uranium. Thus, in order to provide a satisfactory etching rateas well as uniform etching, the concentration of the ferric chloride inthe etchant is preferably in a range of about 4.8 to 5.5 moles per literof water. At higher FeCl concentrations than those in the preferredrange, the etching rate slows down until it is virtually non-existent ina saturated solution at 30 C. However, with FeCl concentrations lessthan about 4.8 M, the etching rate and the temperature of the etchingsolution significantly increase until the etching rate is almostcatastrophic at a concentration of approximately 4.1 M. Further dilutionof the etching bath below 4.1 M effects a decrease in the etching rateuntil it is again in the practical range. While etching solutions ofthese lower FeCl concentrations may be usable it has been found thatthese solutions do not provide the uniformity in the etched surface ordependability of the etching solutions with FeCl concentrations in thepreferred range.

The etching solution of the present invention is not dependent upon theuse of elevated temperatures such as required of many previouslyutilized etchants in that excellent results have been achieved in arelatively low temperature range of about 3060 C. As shown in FIG. 2,the etching rates of a 5.2-molar FeCl solution are fairly uniformbetween 30 and 60 C. However, at temperatures below about 30 C., theetching solution tends to become viscous, which may cause somecrystallization to take place and thereby impair the etching propertiesof the solution. At temperatures greater than about 60 C., on the otherhand, the etching rate increases rapidly and detracts from the desirableetching rate provided by the cooler solutions.

Perhaps one of the more desirable features of the aqueous FeCl etchingsolution is the control factor provided by the relatively slow etchingrate which permits close regulation over the surface finish and thequantity of material removed by the etchant. When employing the etchingsolution of the present invention, in the preferred concentration rangethe etching rate is fairly rapid when the uranium piece is firstimmersed in the etching bath but immediately slows down, and afterapproximately two minutes the etching rate is considerably slower sothat a total etching time of about to minutes is required for effectinga desirable surface finish on the uranium piece. It has been found thatthe surface finish of a uranium piece is properly etched when thequantity of material removed from the surface corresponds to a depth ofabout 0.6 to about 1.0 mil. The characteristics (centerline averages inmicroinches) of the surface finish on uranium pieces etched in a5.2-molar ferric chloride etching bath are more clearly illustrated bythe following table.

TABLE I.SURFACE FINISH OF URANIUM E'ICHED IN CHLORIDE SOLUTIONS Surfacefinish Etching conditions (microinches OLA) Tern era- Time Before AfterEtchant ture( C.) (minutes) etching etching The mixed chloride solution,i.e., the nickel chlorideferric chloride solution briefly describedabove, affords satisfactory etching results, but may not be quite asdesirable as the ferric chloride solution in that the rate of uraniumremoval in the mixed chloride bath is substantially greater than that inthe ferric chloride bath. The concentration of the chlorides in thismixed bath has been found to be most desirable when the solution isprepared with about 3.0 M NiCl and 1.5 M FeCl As shown in the followingtables, the concentration of the mixed chloride solution has aconsiderable effect upon the rate of material removal.

TABLE III.EFFECT OF TEMPERATURE ON THE RATE OF ETCHING OF URANIUM INNICKEL CHLORID RIC CHLORIDE SOLUTIONS Molar concentration Temper-Material Etch time ature removed Ni Fe (minutes) C.) (mils) TABLEII.EFFECT OF CONCENTRATION ON THE RATE OF ETCHING OF URANIUM IN A NICKELCHLORIDE- FERRIC CHLORIDE SOLUTION [3.0 M NiClz+1.5 M FeCla] MaterialEtch time removed Temperature C.) (minutes) (mils) 4 faces by immersingthe etched uranium in a suitable iron solvent such as nitric acid.

In order to provide a better understanding of the present invention anexample of a typical etching operation on metallic uranium is set forthbelow. In this example the the uranium is preferably cleansed ofcontaminants and oxides prior to the etching treatment by utilizingconventional cleaning mechanisms such as grease solvents, detergents,sand blasting, removal of oxides with acids, etc.

EXAMPLE A metallic uranium disk was initially prepared for etching inthe manner described above and then immersed in a ferric chlorideetching solution prepared by admixing 1.4 grams ferric chloridehexahydrate per milliliter water (5.2 M FeCl concentration). After theuranium disk was in the solution for 16 minutes with the latter at 30C., the disk was removed and rinsed with distilled water. The disk wasthen pickled in a nitric acid solution (35 weight percent at roomtemperature) for a duration of 8 minutes to remove iron smut formed onthe surface. The material removed from the surface of the uranium diskwas 0.588 mil, the average rate of removal being 0.037 mil per minute.The etched uranium disk was then provided with an electrolyticallydeposited nickel plate.

The ferric chloride etching solution of the present invention produces auniformly etched surface on metallic uranium which is fairly smooth butsufiiciently rough to effect the required mechanical bonding with thenickel plate. In fact, the etching-solution of the present inventiontends to improve already rough surfaces rather than increase theroughness of such surfaces. Inasmuch as this novel etching solution hasa significantly slower etching rate than previously known etchants, theoperational control factor substantially expedites the formation ofuniformly etched surfaces. Additionally, the novel bath of the presentinvention exhibits a significantly longer useable life than the previousetchants due to a higher tolerance for uranium concentrations in thesolution and a lower dependence of the etching rate upon temperature.For example, satisfactory etching of metallic uranium is accomplishedwithout objectionable variances in the etching conditions even whenuranium concentrations in excess of 10 ounces per gallon of solution areencountered. The effect of temperature on the rate of etching is smallin the usual operating range of 3060 C., particularly when comparing theeffect of the excessive etching rates encountered by the previouslyknown solutions when subjected to elevated temperatures which are oftenrequired or occur during the etching operation.

A uranium piece etched in the ferric chloride etchant of the presentinvention was nickel plated, given a highhumidity corrosion test (20hours at 86 C. in 100 percent relative humidity), and then recycledtwice to demonstrate the effectiveness of the present etching solution.This recycling consists of stripping off the nickel plate in a cyanidesolution and then cleaning the uranium surface in nitric acid but notre-etching. The part was then replated and corrosion tested again. Nocorrosion was found to be present after any of the plating and exposuretests.

As briefly described above, the ferric chloride etching solution of thepresent invention is useable with uranium alloys; for example, theuranium alloy consisting of 90 percent uranium and 10 percentmolybdenum. A uniform surface etch of this uranium alloy was achieved byusing an etching solution substantially less concentrated than thatdescribed above for metallic uranium. With this alloy the solution foundto be satisfactory consisted of 180 grams of ferric chloride hexahydrateand 140 cubic centimeters of water. The etching was accomplished at 30C. for a duration of approximately 4 minutes. The surface finishprovided on this alloy was -180 microinches CLA. Other alloys of uraniumsuch as uranium-2 percent molybdenum may be similarly etched in the FeClsolution.

It will be seen that the present invention provides a unique etchingsolution which is capable of uniformly etching uranium with minimalmaterial removal without utilizing nitric, hydrochloric, or phosphoricacid. Uranium structures treated with the etching solution of thepresent invention have not shown any failure-tO-etch areas on thesurfaces of the uranium structures, which substantially overcomes theproblem continually encountered in etching operations using previouslyknown etchants. Further, there has been no instance where the parts havenot been etched uniformly.

As various changes may be made in the etching solutions, etchingtemperatures, and etching times without departing from the spirit andscope of the invention and without sacrificing any of its advantages, itis to be understood that all matter herein is to be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. A method of preparing articles of uranium metal and alloys thereoffor the reception of a protective metal coating, comprising the steps ofimmersing the article in an etching bath consisting of a water-chloridesolution with the chloride being selected from the group consisting offerric chloride hexahydrate with the ferric chloride in theWater-chloride solution being in the range of about 4.8 to about 5.5moles per liter or nickel chloride and ferric chloride hexahydrate,maintaining the etching bath at a temperature in a range of about to C.during immersion of the article in the etching bath, maintaining thearticle in the bath for a duration sufficient to uniformly etch exposedsurfaces of the uranium article, removing the article from the bath,rinsing the article in water, and thereafter immersing the article in asolution containing an iron solvent for removing from the articlesurfaces iron values deposited thereon during the immersion in the bath.

2. The method claimed in claim 1, wherein said duration is in a range ofabout 10 to 15 minutes.

References Cited UNITED STATES PATENTS 2,886,420 5/1959 Jones et a1156-18 JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R.

